Transformer voltage regulating system



May 2,- 1939. o. P. M cARTY TRANSFORIER VOLTAGE REGULATING SYSTEM Filed March 20, 1936 2 Sheets- Sheet 1 //3 SUPPLY Fig.2.

Cie/

C13 [as Cas Inventor Orin 1? Mc Cartg,

His Attorneg.

Patented May 2, 1939 PATENT OFFICE TRAN SFOBIWER VOLTAGE BEGULATING SYSTEM Orin P. McCarty, Pittsfield, Mass assignor to General Electric Company, a. corporation of New York Application March 20, 1936, Serial No. 89,789

21 Claims.

My invention relates to transformer voltage regulating systems and more particularly to means for changing the connections'between the taps of a transformer winding for adjusting the voltage ratio, of the transformer.

This application is a continuation in part of my application Serial Number 16,653, filed April 16, 1935, and assigned to the assignee of the present application.

It is usually desirable that the voltage of an electrical circuit be maintained substantially constant under varying load conditions and this is particularly true if the load includes essentially constant potential electrical devices such as incandescent lamps. It is also usually desirable that any voltage regulation of an electrical circuit be accomplished without interrupting the supply of current to the load.

The general object of the invention is to provide an improved transformer tap-changing system which shall be simple in construction and reliable in operation and which can be used to change transformer tap connections without interrupting the transformer load.

Further objects and advantages will be apparent from the following description taken in connection with the accompanying drawings, in which Fig. 1 is a diagrammatic view of a transformer winding provided with tap-changing mechanism constructed and arranged in accordance with the invention; Fig. 2 shows explanatory curves of voltages and currents in various parts of the tap-changing mechanism while a change of tap connections is being made, and Figs. 3, 4 and 5 show modifications of the arrangement shown in Fig. 1.

The transformer winding shown in Fig. 1 includes a common winding l0, a series winding II and a voltage winding l2. This transformer winding is connected between a primary or supply circuit I3 and a load or secondary circuit I4. The circuit I4 is connected across the common winding l0 and one side is also connected to some point of the series winding H such as its center. The series winding II is provided with taps l5,

of the common winding Ill. The movable con--.

tact 2| of the switch I5 is operated by a Geneva gear 22 driven by a motor 23 through a lost motion connection 24, the motor being controlled by a contact-making voltmeter 25 connected to the voltage coil |2 which is of course inductively coupled with the common and series transformer windings I0 and II. If the voltage of the supply circuit rises above its normal desired value, the voltage of the winding I2 will rise and cause the movable contact 26 of the contact-making voltmeter 25 to move onto the fixed contact 35, thus connecting the motor 23 to the voltage winding |2. The motor 23 will then rotate the driving member 28 of the Geneva gear 22 in the direction indicated by'the arrow in Fig. 1. This driving member 28 carries a pivoted arm 29 with a pin 30 arranged to engage slots 3| in the driven member 32 of the Geneva gear. The angular travel of the arm 29 relatively to the driving member 28 is limited by pins 33. Rotation of thedriving member 28 tensions a spring 34 until just as the pin 30 reaches one of the slots 3| in the driven member 32 when the spring will act to further rotate the driving member 23 rapidly through approximately a half revolution and the driven member 32 a sin'glestep in the same direction. This will rapidly transfer the movable contact 2| of the switch I5 to an adjacent contact 20 in a direction to restore the voltage of the load circuit l4 toward its normal value. If the voltage of the primary circuit |3 drops below its desired normal value, then the movable contact 26 of the contact-making voltmeter 25 will move onto the contact 21 of the contactmaking voltmeter, thus connecting the motor 23 to the voltage winding l2 and causing the motor to rotate the driving member 28 of the Geneva gear 22 in a direction opposite to that indicated by the arrow'in Fig. 1. This will act in the manner already described to transfer the movable contact 2| of the switch I5 rapidly to an adjacent fixed contact 20 so as to raise the voltage of the load circuit towards its desired normal value.

If the movable contact 2| of the switch I5 is transferred from one fixed contact 20 to another while the load current is flowing in the circuits I3 and I4, there would tend to be serious arcing between the contacts and the contacts would thus soon become seriously burned and. damaged. To prevent this serious arcing and damage, a by-pass 36 is connected between the movable contact and the series winding, the connection to the series winding being shown at the center or electrical mid-point of the winding. This by-pass 36 may be any suitable impedance, one satisfactory form of' which is a resistor which has a normally high resistance but a high negative voltage-resistance characteristic so that its resistance will decrease considerably as the voltage across it is increased and will increase considerably as the voltage across it is decreased. A suitable material for this resistor is disclosed in the United States Patent No. 1,822,742, granted September 8, 1931, upon an application of K. B. McEachron and assigned to the assignee of the present application.

When the movable contact 2| of the switch 15' is connected to the same tap to which the resistor 36 is connected, then it is obvious that the resistor 36 is short-circuited by the switch l5. Under these conditions the resistance of the resistor 36 will be very high, the current through the resistor will be negligible and substantially all of the load current will flow through the switch 15'. When the movable contact 2| is connected to any other tap than that one to which the resistor 36 is connected, then the voltage between these two taps will be impressed across the resistor but this voltage is small and the current through the resistor will therefore be substantially negligible, substantially all the load current still flowing through the switch I5. When the movable contact 2| is, however, between two of the fixed contacts 20, then the circuit through the switch I 5' is open and the full circuit or transformer voltage will be impressed across the resistor 36. With this relatively very high voltage impressed across it, the resistance of the resistor 36 will instantly decrease to a very low value and afiord an easy path through the resistor for the load current, thus quickly extinguishing any are and preventing damage at th switch contacts.

Assume that the movable contact 2| is connected to the same tap I! to which the resistor 36 is connected and that the movable contact is to be transferred rapidly to the adjacent fixed contact 26 which is connected to the tap l8 as indicated in Fig. 1. The curves shown in Fig, 2 indicate the relative-voltage and current conditions in different parts of the tap-changing mechanism which has been described. The broken line 31 of Fig. 2 indicates the instant at which the movable contact 2| leaves the first fixed contact 20 and the line 38 indicates the instant at which the movable contact 2| reaches the second fixed contact 20 to complete a change in tap connections. The curve C39 indicates the current in 'the tap connection 39 from ,the tap B8. The

curve C40 indicates the current in the tap connection 46 from the tap H. The curve C21 lnd1- cates the current in the movable contact 2i of the switch IS. The curve C13 indicates the cur-' rent in the supply circuit l3. The curve E36 indicates the voltage across the resistor 36. The curve C36 indicates the current in the resistor 36. The curve 013 shows that the current in the supply circuit l3 flows continuously and is not interrupted while a change in tap connections is being made. The curves C39, C4o,"C21 and C36 show that the current flows inthe movable contact 2| and the tap connection 40 until the first instant at which the current reaches a zero value after the movable contact 2i has brokenits connection with this tap connection 46. The current is then transferred to the resistor 36 until the movable contact 2! reaches the tap connection 39 at which instant the current is transferred from the resistor to this tap connection 39. The curve E36 shows that the voltage across the resistor 36 is at least small enough to be negligible except while the currrent is broken through the switch I5.

The invention provides a very simple and reliable tap-changing mechanism, the provision of the by-pass resistor 36 permitting a very simple form of switch to be used for changing the tap connections without interrupting the current in the load circuit. It is desirable that the switchoperating mechanism and the distance between the fixed contacts 20 be so arranged and proportioned that the load current normally flowing through the movable contact 2| will always have sufiicient time to reach a zero value after the movable contact 2| has left one fixed contact 26 and before it reaches an adjacent contact 26. This will assume a transfer of the load current to the resistor 36 and acomplete extinguishment of any are between the contacts before the transfer of the movable contact has been completed.

By connecting, the by-pass impedance 36 to the mid-point of the "series winding, the maxi mum continuous voltage across it, is limited to half the voltage of the series winding.

The modification shown in Fig. 3 is particularly well adapted for use as a branch feeder circuit step voltage regulator. In general principle it is similar to Fig. 1 but it differs therefrom in the following particulars. The primary relay or contact-making voltmeter 25 is provided with holding coils M and 42 which are connected so as to be deenergized when the floating contact 26 is not in engagement with the stationary contacts 21 and 35 but which are energized, respectively, when the floating contact 26 engages the fixed contact 27 or the fixed contact 35. These.holding coils may be energized in any suitable manner and as shown they. are connected in parallel with the motor energizing circuits. The purpose of these holding coils is to insure a firm non-chattering or arcing contact between the fixed contacts and the floating contact 26. Thus, for example, wherrthe floating contact 26 engages the' stationary contact 21 the holding coil 4| is energized, thereby magnetically holding the contact 26 in engagement with the contact 21 and preventing a trembling or vibratory engagement between these contacts, which type of engagement produces injurious pitting and burning of the contact-making voltmeter contacts. Similarly, when contact 26 engages fixed contact 35 the holding coil 42 is energized so as to insure a firm and positive engagement between the contacts. Another result of the holding coils is to cause the contacts of meter 25 to open and close at difierent voltage values. For example, the voltage resulting in the engagement of contacts 26 and 21 is lower than the voltage required for this separation because of the pull of coil 4|.

It will also be. noted that the series, secondary, or regulating winding H is connected in the supply circuit side of the common or primary-winding H1, or in other words that the common winding I0 is always connected directly across the load side of the circuit l4. Consequently, as the number of turns of the common winding across the load circuit is constant the voltage induced in the auxiliary winding i2 is always a direct measure of the load circuit voltage. This is also true of the relation between the windings I0 and I2 of Fig. 1.

The next difference between Fig. 3 and Fig. 1 is that in Fig. 3 the circuit is provided with a manually operable selector switch or terminal board 43. By means of this additional switch it is possible to have what may be described as a "five in one regulator. Thus the regulator of Fig. 1 only produces two steps of voltage increase or boost and two steps of voltage decrease or buck and the amounts of buck and boost are equal. However, in Fig. 3 by means of the connection changing switch 43 it is possible to provide selectively five difierent ranges of regulation. Switch 43 is provided with a movable contact 44 which,

as shown, is contacting a fixed contact 46. In

this position, one side of the load circuit is connected directly to the outer end of the series winding II. If the voltage of the entire series winding H is 10% of the circuit voltage then the illustrated position of the switch 43 may be termed the 10% boost position because operation of the tap changing movable contact 2| over the contacts 20 can produce a range of regulation from zero to a maximum of 10% boost. In the position in which contact 2| is shown in Fig. 3 a 5% boost is being produced and if the contact 2| is moved counter-clockwise from this position it will successively produce a 7 /2% voltage boost and a voltage boost, while if it is moved in a clockwise direction it will produce successively a 2 voltage boost and zero boost as it successively engages the two fixed contacts 20.

The 5% boost which is produced with the parts in the position shown in the drawings is due to the fact that the load current flows from the lower conductor of the supply circuit I3 through the movable contact 2|, the tap I! on the series winding II, the lower half of the series winding which produces a 5% voltage boost, then through the fixed contact, movable contact 44 and out to the load circuit through the lower conductor of the load circuit l4. The return current path, of course,'is through the upper conductors of the load and supply circuits l4 and I 3.

If now the movable contact 44 of the connection changing switch 43 is moved clockwise to the adjacent fixed contact 46 the range of regulation which can be secured by the tap changing movable contact 2| will be from a maximum of '7 boost through zero to a maximum of a 2 voltage buck. With the contact 2| in the position shown there will be a 2 voltage boost produced, while if the contact M is moved clockwise the boost will first be reduced to zero as the first fixed contact 10 is engaged and as the next contact 20 is engaged there will-be a 2 /2% voltage buck produced. This is because when the movable contact 2| is engaging the second removed clockwise contact 20 the load current flows through the section of the series winding H between the taps l9 and IS in a direction in opposition to the voltage of this portion of the winding thereby producing a voltage buck.

If the movable contact 44 of switch 43 is moved to the next adjacent contact 41 the tap changing circuit will be the equivalent of that shown in 'Fig. 1 and 5% boost and 5% buck will be produced in equal 2 /2% steps. This is because the load circuit will then be connected directly to the midpoint of the winding II as in Fig. 1. If the movable contact 44 is moved to the next contact 48 it will be possible to secure a 2 voltage boost and a l voltage buck in 2 steps. Finally if the movable contact 44 is moved into engagement with the stationary contact 49 the connection will be the reverse of that shown in the drawings and it will be possible to secure a 10% voltage buck in four 2 steps.

Ordinarily, the range selecting switch 43 will be set at the position giving the range of regulation which is desired for any particular location and circuit in which the regulator is used and in order to change the range of regulation it will be necessary to disconnect the regulator from the circuit so that no current is flowing through the switch 43. However, this is not necessarily essential and if a load current by-pass impedance similar to resistor 36 is connected in shunt with the switch 43 this switch may be operated to change the range of regulation while the regulator is carrying load current. For example, the resistor 50 may be taken as such a resistor. However, resistor 50 serves another purpose whichis described immediately below and preferably is not designed to carry the load current of the circuit as this would unnecessarily increase the cost of the regulator.

The primary purpose of resistor 50, which ispreferably made of the same material as the resistor 36, is to complete a protective by-pass circuit or shunt around the series winding I l. Very often lightning and transient voltages produced during switching cause such high voltages as would possibly injure the insulation of the series winding l i, as these high voltages travel in surges along the circuit. However, if such a high voltage surge reaches the regulator the resistors 36 and 50 in series completely shunt or by-pass the series winding II and due to their characteristic of instantaneously reducing their resistance with increases in applied voltage they form a protective non-linear by-pass for preventing excessive voltages across the series winding II.- It will, of course, be seen that resistors 36 and 50 are, in

fact, connected in series across the series winding. H and directly interconnect the supply circuit rates, producing a relatively low efiective impedance by-pass. The highvoltage peaks produced as the reactor current is passing through zero and which consequently are not reduced by saturation cause the non-linear resistance 36 to decrease and therefore the resistor acts as a voltage limiting device -for the reactor. Such a combination, for a given performance, is consider-.- ably less expensive and more practical than a non-linear resistor alone because a saturable reactor which is capable of carrying the load current can be produced for less cost than a nonlinear resistance of the same current rating.

In Fig. 4 a capacitor 53 is connected in parallel with the load by-pass resistor 36. The action is similar to the action of a saturable reactor in that after the arcing current has passed through zero and starts rising the capacitor forms a low impedance to the rising current thereby consuming the current and allowing the arc to become extinguished. As the capacitor becomes charged, the voltage across it rises to a value which will cause the resistance of the non-linear resistor 36 to decrease. This non-linear resistor then, forms a low impedance path for the flow of current until the moving contact 2| reaches the next fixed .voltage boost.

Like Figs. 1 and 3, it consists p'rimarily'of an auto-transformer having a common winding l0 and a series winding H. An electro-magnetically operated contactor 54 of the two-position type is provided with two sets of contacts 55 and 56, only one of which sets can be closed at one time. When contacts 55 are closed and contacts 56 are open the series winding H is completely out of circuit and the load current flows through the contacts 55. When the contacts 56 are closed and the contacts 55 are open the entire series winding II is connected in the load circuit through conductors 51, 58 and contacts 56.

During thepositional change of the contacts the load current by-pass impedance 36 carries the current and as shown this impedance is connected from an'intermediate point in the series winding H to the load circuit H. The operation of tap changing is therefore similar to the corresponding operation of Figs. 1 and 3.

The booster of Fig. 5 is also provided with an over-voltage protective by-pass 50 which in this case is connected in shunt to the winding H, or,

more properly, is connected so as directly to interconnect the supply circuit [3 and the load circuit l4.

Aside from the contactor 54 the only other movable part is the armature and movable contact of a relay 59 forming a part of what may be described as a resonant relay control circuit.

This resonant relay control circuit consists of a capacitor 60, an adjustable saturable core reactor 10 and a resistor H connected in series through a switch 12 across a part of the series winding I II As the voltage of the series winding is directly proportional to the load circuit voltage the voltage applied to the resonant relay circuit is a direct measure of the load voltage. This circuit, including the capacitor 50 and the reactorlfl, is capable of being tuned or becoming resonant, it being so arranged that it becomes resonant at a value of voltage determined by the'adjustment of the reactor 10 and becomes dissonant at a lower voltage determined by the setting of the adjustable resistance 1|. Briefly, the reason that the resonant phenomenon is responsive to voltage is because changes in voltage change the current flowing through the resonant circuit thereby changing the magnetic saturation of the owed the reactor which in turn changes the inductive value of the reactor and at a particular inductive value corresponding to the'capacitive value of the capacwhich is lower than the voltage causing res-.

onance.

The contacts of the relay 59 are connected across a portion of the series winding H in series with the alternating current input terminals of a rectifier l3 whichmay be of any well known type and is shown by way of example as a bridge connected copper oxide rectifier.

The direct current output terminals of the rectifier 13 are connected to the operating coil of the contactor 54.

In the operation of Fig. 5, if the switch 12 is closed and the voltage of the load circuit is too low to cause resonance then the contacts of relay 59 will be closed thereby energizing the contactor 54 and causing'it to close its contacts 56 and open itscontacts 55. This thereby connects the series winding H in the power circuit and produces a definite predetermined voltage boost. If now the voltage of the power circuit should be too high resonance will occur thereby causing the relay 5!! to open its contacts whereby the contactor 54 is deenergized and a strong spring 14 will rapidly snap the contacts 55 closed and open the contacts 56 thereby entirely disconnecting the series winding I I from the circuit.

By means of the adjustments of the reactor 10 and the resistor II it is possible, if the series Winding II produces a 10% voltage boost, to adjust the control circuit so that a 10% voltage boost is produced whenever the voltage of the load circuit drops to volts, for example, thereby bringing it up to 121 volts and toretum the contactor to the no-boost position when the voltage reaches 123 volts where it will remain in a no-boost position until the voltage again drops to 110 volts.

While I-have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that changes and modifications can be made without departing from my invention and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope or my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In combination, an alternating current circuit, a transformer including a winding having taps, a ratio changing switch having a transformer side connected to said taps and having a circuit side connected directly to said circuit independently of any part of said winding, and aload current bypass connected between the electrical midpoint of said winding and the circuit side of said switch, said bypass being" connected and proportioned to carry the full load current of said winding during a positional change of said switch.

i 2. A transformer including a winding having taps, a switch having fixed contacts connected to said taps and having but one movable member for successively engaging said fixed contacts so as to change the voltage ratio of said transformer, and an impedance connected to bypass said movable member, said impedance having one end thereof connected to an intermediate point on said winding and being proportioned to carry the full load current of said winding during operation of said switch, said point on said winding being difierent from the points where current normally enters and leaves said winding whereby the current. through said impedance passes through a portion of said winding.

3. In. combination, an alternating current circuit, a transformer winding one end of which is permanently connected to said circuit, a two position ratio changing switch having a winding side and a circuit side, said switch being arranged so that in one position it completes a circuit connection through said winding and in another position it interrupts said connection and bypasses said winding, and an-impedance connected between substantially the electrical midpoint of said winding and the circuit side of said switch, said impedance being proportioned to carry the full load current of said winding during the positional change of said switch;

4. A transformer including a winding having taps, a switch having a movable contact member for successively making connection to said taps so as to change the voltage ratio of said transformer, and a nonlinear resistor connected to bypass said switch and being proportioned to,

carry the full load current of said winding during operation of the switch.

5. A transformer including a winding having taps, a switch having a movable contact member for successively making connection to said taps so as to change the voltage ratio of said transformer, and a negative resistance current characteristic impedance connected to bypass said switch and being proportioned to carry the full load current of said winding during operation of the switch.

6. A transformer including a winding having taps, a switch connected to said taps for changing the voltage ratio of the transformer, and normally high resistance means connected to bypass said switch, said means having a high nega-. tive voltage-resistance characteristic and being proportioned to carry the full loadcurrent of said winding during operation of the switch.

7. A transformer includingaJ-winding having taps, a switch connected to said taps for changing the voltage rattio of the trransformer, and a normally high resistance resistor connected to by-pass said switch, said resistor having a high negative voltage-resistance characteristic and being proportioned to carry thefull load current of said winding during operation of the switch.

8. A transformer including a winding having taps, a switch connected to said taps, means for operating said switch rapidly to change tap connections, and normally high resistance means connected to by-pass said switch, said by-pass means having a high negative voltage-resistance characteristic and being proportioned to carry the full load current of said winding during operation ofthe switch, and said switch and switch operating means being proportioned and arranged to assure a zero value of current through the switch during the period of its operation.

,9. A transformer including a winding having taps, a switch having fixed contacts connected to said taps, said switch also including a movable contact, means for moving said movable contact rapidly from one fixed contact to another to change the voltage ratio of the transformer, and a normally high resistance by-pass 7 connected between said movable contact and said 'taps so as to change the voltage ratio of said transformer, and an impedance network having a plurality of impedances with different voltampere characteristics connected between said movable contact member and said winding and being proportioned to carry the full load current of said winding during operation of said switch. 11. A transformer including a winding having taps, a switch having a movable contact member for successively making connection to said taps so as to change the voltage ratio of said transformer, and an impedance network connected between said movable contact member and said winding and being proportioned to carry the full load current of said winding during operation of said switch, said impedance network comprising a resistance element and a reactance element connected in parallel. I

12. A transformer including a winding having taps, a switch having a movable contact member for successively making connection to said taps so as to change the voltage ratio of said transformer, and an impedance network connected between said movable contact member and said winding and being proportioned to carry the full load current of said winding during operation of said switch, said impedance network comprising a nonlinear resistance-voltage characteristic resistance and an inductive winding connected in parallel. Y

13. A- transformer including a winding having taps, a switch having a movable contact member for successively making connection to said taps so asto change the voltage ratio of said transformer, and an impedance network connected between said movable contact member and said winding and being proportioned to carry the full load current of said winding during operation of said switch, said impedance network comprising a negative resistance-voltage characteristic resistance and a saturable core reactor connected in parallel.

14. Voltage adjusting means for an electrical circuit comprising a transformer having a secondary winding adapted to carry the circuit load current and a primary winding energized by the circuit voltage, a two position electromagnetically actuated spring returned contactor adapted in its actuated position to connect said secondary winding in series circuit relation with the circuit and in its returned position to interrupt this connection and bypass said secondary winding, a primary relay sensitive to changes in an electrical characteristic of the circuit for controlling actuation of said contactor from one to the other of the two positions above named, and negative impedance-current characteristic impedance adapted to carry the circuit load current during the period of contactor positional change.

15. In combination, a transformer including a winding having taps, a switch connected to said taps for changing the voltage ratio of said transformer, a connection changer connected to said taps for changing the range of adjustment of the voltage of said winding, a switch current bypass connected around said switch, and an overvoltage protective device connected around said connection changer.

16. In combination, an alternating current circuit, an autotransformer having ,a series winding provided with taps, a ratio changing switch connected to said taps and having a movable contact connected to said circuit on one side of said winding, a switch current by-pass impedance connected around said switch, a connection changing range adjuster connected to said taps and having a movable contact connected to said circuit on the other side of said winding, and a negative resistance-voltage characteristic resistance connected as an overvoltage protective means around said connection changing range adjuster.

17. A transformer including a winding having taps, switching means for successively making in-- dividual connection to said taps so as to change the voltage ratio of said transformer, said switching means having apparatus which interrupts the circuit therethrough without making a bridging connection between any pair of taps during a tap changing operation, and non-linear impedance means connected to by-pass said switch and being proportioned to carry the full load current of said winding when said switch is open, said nonlinear impedance means being continuously conductive at all values of voltage thereacross and having a percentage change in voltage drop which isalways less than any percentage change in load current therethrough.

18. A transformer including a winding having taps, switching means for successively making individual connection to said taps so as to change the voltage ratio of said transformer, said switch- 1 ing means having apparatus which interrupts the circuit therethrough without making a bridging connection between any pair of taps during a tap changing operation, and non-linear impedance means connected to by-pas's said switch and being proportioned to carry the full load current of connection between any pair of taps during a tap,

changing operation, and a non-linear resistor connected to by-pass said switch and being proportioned to carry the full load current of said winding during operation of said switch, said resistor being continuously conductive at all values of voltage thereacross and having a resistance which varies instantaneously inversely with variations in the instantaneous value of the current therethrough.

20. A transformer including a winding having taps, switching means for successively making individual connection to said taps so as to change the voltage ratio of said transformer, said switching means having apparatus which interrupts the circuit therethrough without making a bridging connection between any pair of taps during a tap. changing operation, and non-linear impedance means connected to by-pass said switch and being proportioned to carry the full load current of said winding during operation of said switch, said non-linear impedance means being continuously.

conductive at all values of voltage thereacross and'including a pair of separate impedance elements connected in parallel circuit relation, one of said elements having animpedance which varies inversely with the current therethrough, the other of said elements having an impedance which varies in accordance with the rate of change of current therethrough.

,21. A transformer including a winding having taps, switching means for successively making individual connection to said taps so as to change the voltage ratio of said transformer, said switching means having apparatus which interrupts the circuit therethrough without making a bridging connection between any pair of taps during a tap changing operation, and non-linear impedance means connected to by-pass said switch and 

